1. Field of the Invention
This invention relates to a magnetic radial force actuator and more particularly to a radial magnetic bearing for applying a magnetic force to a rotor within a stator assembly.
2. Description of the Prior Art
Magnetic bearings for rotatably supporting a rotor about an axis as well as to freely translate the rotor about its axis are well-known in the art as disclosed in U.S. Pat. No. 4,473,259. It is the principal advantage of a magnetic bearing to support the shaft in a manner that eliminates frictional wear and the corresponding need for lubricants.
A conventional magnetic bearing includes a series of electromagnets equally spaced around a rotor member having a ferrous outer ring in close proximity to the poles of the electromagnets. Forces are exerted on the rotor member by supplying currents to the coils wrapped around the poles of the electromagnets. It is also known that by adjusting the current in each electromagnet the forces exerted on the rotor can be brought into balance so that the rotor can be maintained in a levitated state.
A further example of a radial magnetic bearing is disclosed in U.S. Pat. No. 4,121,143 in which a rotor is supported in a stator by two radial magnetic bearing means, each having an electromagnet with eight windings mounted on the poles of the fixed armature. There is no contact between the rotor and the stator and therefore there is no friction. However, the flux orientation in the rotor undergoes a number of reversals in each revolution of the rotor. This results in a certain degree of resistance to motion by generating hysteresis losses in the rotor as the flux reverses in each element of the rotor as it passes from pole to pole. In addition, losses due to eddy currents induced in the rotor also have a retarding effect on the rotation of the rotor.
It has been suggested as disclosed in U.S. Pat. No. 4,012,083 to reduce the rotation retarding effects on the rotor associated with hysteresis and eddy current losses in a magnetic bearing by utilizing in the electromagnetic circuit a stator having at least one electromagnet of U-shaped cross section with the arms of the U forming north and south poles where the plane of symmetry separating the north and south poles lies perpendicular to the axis of rotation of the magnetic bearing.
While attempts have been made to substantially eliminate the frictional drag forces that are experienced with magnetic bearings due to hysteresis and eddy current losses in the rotor, the proposed construction of the rotor and stator elements are elaborate and are expensive to manufacture. Therefore there is need for a magnetic radial force actuator which is economically manufactured and efficient to the degree of substantially eliminating the effects of frictional drag forces on the rotor.